Modern operating systems are required to support many diverse applications having complex interrelationships with the system. Such applications include word processors, spreadsheets, development tools, communications applications, graphical applications, and processing applications to name but a few examples. One innovation for implementing such applications has been through employment of object oriented programming techniques. Object oriented programming shifts the emphasis of software development away from function decomposition and towards the recognition of units of software called “objects” which encapsulate both data and functions. Object Oriented Programming (OOP) objects are software entities comprising data structures and operations on data. Together, these elements enable objects to model virtually any real-world entity in terms of its characteristics, represented by its data elements, and its behavior represented by its data manipulation functions. In this way, objects can model concrete things like people and computers, and they can model abstract concepts like numbers or geometrical concepts. For example, an “object-oriented” computer scientist would be mostly concerned with an application under design, and secondarily with the tools used to develop it—whereas a “non-object-oriented” scientist would think primarily of his or her tools.
The benefit of object technology arises out of three basic principles: encapsulation, polymorphism and inheritance. Objects hide or encapsulate the internal structure of their data and the algorithms by which their functions work. Instead of exposing these implementation details, objects present interfaces that represent their abstractions cleanly with no extraneous information. Polymorphism takes encapsulation one step further—the idea being many shapes, one interface. A software component can make a request of another component without knowing exactly what that component is. The component that receives the request interprets it and figures out according to its variables and data how to execute the request. The third principle is inheritance, which allows developers to reuse pre-existing design and code. This capability allows developers to avoid creating software from scratch. Rather, through inheritance, developers derive subclasses that inherit behaviors which the developer then customizes to meet particular needs.
In particular, an object includes, and is characterized by, a set of data (e.g., image data) and a set of operations (e.g., methods), that can operate on the data. Generally, an object's data is ideally changed only through the operation of the object's methods. Methods in an object are invoked by passing a message to the object (e.g., message passing). The message specifies a method name and an argument list. When the object receives the message, code associated with the named method is executed with the formal parameters of the method bound to the corresponding values in the argument list. Methods and message passing in OOP are analogous to procedures and procedure calls in procedure-oriented software environments.
However, while procedures operate to modify and return passed parameters, methods operate to modify the internal state of the associated objects (by modifying the data contained therein). The combination of data and methods in objects is called encapsulation. Encapsulation provides for the state of an object to only be changed by well-defined methods associated with the object. When the behavior of an object is confined to such well-defined locations and interfaces, changes (e.g., code modifications) in the object will have minimal impact on the other objects and elements in the system.
Each object is an instance of some class. A class includes a set of data attributes plus a set of allowable operations (e.g., methods) on the data attributes. As mentioned above, OOP supports inheritance—a class (called a subclass) may be derived from another class (called a base class, parent class, etc.), where the subclass inherits the data attributes and methods of the base class. The subclass may specialize the base class by adding code which overrides the data and/or methods of the base class, or which adds new data attributes and methods. Thus, inheritance represents a mechanism by which abstractions are made increasingly concrete as subclasses are created for greater levels of specialization.
Although object-oriented programming techniques are still widely employed, newer systems and applications have evolved. One such system includes a managed object system whereby the operating system itself is involved in the determination and management of the lifetime of an object (e.g., automatically reclaiming an object from system memory via a garbage collector versus internal reference counting on the object). These systems also support different types of data such as meta-data, for example, that provide data and corresponding descriptions for such data. When newer applications are developed for these type data structures and systems however, object-oriented models are at a minimum are problematic. For example, such models may require application designers to maintain and manage operating system states and events in order to successfully implement an application. Thus, with traditional approaches, even simple changes to an application document or item can require very complex code to maintain internal document structure and to support corresponding changes.